A considerable number of primordial radioisotopes are present in almost all the samples extracted from the Earth’s crust, such as oil, rock, soil or other materials. Their concentrations are often determined by gamma spectrometry. Although the relative concentrations of isotopes often fluctuate within a narrow range, it is not always the case. Some natural materials (such as naturally occurring radioactive material) show unusual activity ratio between 238U and 232Th, while technologically processed materials (technologically enhanced naturally occurring radioactive material) might also introduce significant disequilibrium in radioactive chains. Knowing that primordial radioisotopes emit in total more than a thousand gamma and characteristic X-ray photons and that many of them interfere with each other, a question arises whether for some activity ratios commonly used photopeaks become useless for quantitative analysis, due to interferences with other photopeaks. A computer program was developed in order to calculate full energy photon interferences for any chosen photopeak. The calculations are based on the inputs in the form of isotope activities and detector calibration equations and its characteristics are presented in this paper. [Projekat Ministarstva nauke Republike Srbije, br. III43009]